Powering the Fantastic Voyage with Nanomotors

By Chris Palmer

San Diego, Calif., Nov.17, 2011 — In 1966, “The Fantastic Voyage” captured filmgoers’ imaginations with scenes of scientists careening through a person’s bloodstream in a miniaturized ship. The latest nanomolecular tools coming out of UC San Diego are bringing aspects of this classic science fiction tale closer to reality.

Joseph Wang

Along with his colleagues, Joseph Wang, a professor in the Department of NanoEngineering in UC San Diego’s Jacobs School of Engineering, has developed a variety of nanomolecular tools called ‘nanomotors’ that can cruise through biological samples such as blood, urine and saliva. These devices can capture, separate and purify biological targets, making them relatively easy to detect and quantify, all at a fraction of the cost of current laboratory protocols in terms of time, labor and money.

“There are many possible applications, including disease diagnosis, cancer cell detection, detection of food pathogens, nanofabrication and nanoassembly,” says Wang, who is based in Atkinson Hall and sits on the Executive Council of the California Institute for Telecommunications and Information Technology (Calit2) at UCSD.

Much of Wang’s previous work has focused on biosensors – handheld devices that contain a series of wells into which biological samples can be deposited. Each well has a unique DNA probe or antibody that produces an electrical signal when the appropriate target is detected.

Wang is now combining his new expertise, nanomotors, with his pre-existing expertise in biosensors.

“We thought, why not have the antibody actively moving in the blood and picking up target proteins? This adds a completely new dimension to sensing and diagnostics.”

Wang, along with research scholar Susana Campuzano, graduate student Daniel Kagan and postdoctoral scholar Jahir Orozco (who are all from UC San Diego’s Department of NanoEngineering), recently published a peer-reviewed paper in the journal Analyst reviewing the features of their current generation of nanomotors.

Hydrogen peroxide-powered nanowires with specialized receptor probes capture biological targets like bacteria. The total distance that the nanowire travels through a biological sample like blood or saliva depends on how much bacteria it captures. The distance the nanowire travels indicates how many colony forming units (CFU) of E. coli bacteria were present in each of the three biological samples.

Among Wang’s latest nanomotor creations -- which are named for their nanometer (one-billionth of a meter) size range -- are ‘nanowires’ and ‘microrockets’ that can operate in biological samples. Both use hydrogen peroxide as fuel and both can be ‘steered’ with magnetic fields.

Nanowires are rod-like structures, two to three micrometers in length, composed of gold on one end and platinum on the other end. The platinum end reacts chemically with hydrogen peroxide, a small amount of which can be added to a biological sample, to create a stream of electrons that then travel through the nanowire towards the gold end. This asymmetric flow of electrons produces a propulsive force that propels the nanowire forward at up to 100 micrometers per second. A magnetic field can be used to steer the nanowire in a straight line.

Specialized receptor probes can be attached to the nanowire. When the probes capture specific target biomolecules such as nucleic acids or bacteria, another chemical reaction is facilitated that increases the speed of the nanowire and, ultimately, the distance it will travel. Therefore, when placed in a biological sample, the distance that a nanowire travels is an indicator of the amount of the target biomolecule that is present.

Microrockets are conical, tube-like structures, 50 to 100 micrometers in length, with platinum lining the tube’s inner surface. As with the nanowires, the platinum catalyzes the decomposition of the peroxide fuel, creating bubbles of oxygen that expand as they move down the length of the widening tube. The unidirectional flow of bubbles creates a propulsive force, powering the microrocket at speeds of up to two millimeters per second.

Microrockets can also be outfitted with specific receptor probes that, as the microrocket migrates through a biological sample, grab onto and transport specific biomolecules. Microrockets can tow their cargo relatively large distances into a clean environment for the purpose of isolation, separation and purification. Large cargo, such as cancer cells, can then be directly identified with an optical microscope.

Microrockets, propelled by a chemical reaction that creates bubbles of oxygen that stream out the back end, pick up and transport a specific type of cancer cell (CEA+) to a clean environment where it can be isolated and identified.

“Our inspiration for these synthetic nanomotors comes from biological nanomotors like kinesin,” says Wang of the ubiquitous cellular protein, which uses the energy of adenosine triphosphate (ATP) to transport cargo, such as vesicles filled with hormones, in a ‘hand-over-hand’ manner along filaments within cells.

Though nanomotors are not yet ready for use in living organisms, Wang believes they are ready to be incorporated into ‘lab-on-a-chip’ devices. These devices are microchips etched with various channels and compartments that can be filled with biological samples. Nanomotors attached to specialized receptors can be guided through the channels, picking up target molecules and transporting them to isolation compartments for analysis and quantification, potentially providing a simple, low-cost solution to medical diagnosis and biodetection.

“We have a nanomachine ‘factory’ and we are constantly trying to make them better and better,” says Wang who is already working on improvements such as fuel-free propulsion (propulsion would instead be powered by a rotating magnetic field or subtle variations in temperature), coordinated activity of multiple nanomotors, precise motion control, higher towing capacity, faster transport speed and, of course, a decrease in size.